Chemical Damage: Phototoxicity

Symptoms: Browning or yellowing of the leaf margins and brownish or discolored irregular areas on leaves are common symptoms of chemical burns. Severe injury may be evidenced by distortion of new leaves, or defoliation, but unless some serious mistake has been made, spray or dust injury is not an important consideration. Pesticides in emulsifiable formulations are more likely to cause foliage burn than wettable powders, particularly when combined with wettable powders.

Copper: Stone fruits are sensitive to copper. Bordeaux mixture causes russeting of apples and may stunt cantaloupe and watermelon leaves. Copper compounds should be used with caution on any cucurbit crop. Injury is most likely to occur when materials are applied to wet leaves and under damp, slow-drying conditions. Copper containing sprays will also defoliate peaches, plums and apricots.

Zinc Sulfate: This causes severe defoliation of fruit trees and should not be included in fruit sprays.

Sulfur: Frequent applications of sulfur may cause light foliage burn on rose and tender foliaged ornamentals when temperatures exceed 85°F. Sulfur should not be used on cucurbit crops such as squash and cucumbers.

Insecticides: Injury to plants may occur when materials are improperly applied or applied under adverse environmental conditions. Foliage burn is most likely to occur when materials are applied to wet foliage or when extremely high temperatures follow application.

Herbicides: Severe injury to foliage may result from spraying plants with equipment contaminated by previous use with hormone type herbicides. Common symptoms are severe growth abnormalities of leaves and leaf roll. Some herbicides are taken up by plant roots and cause a marginal burn. This burn develops inward to the veins of the leaf as the material is concentrated. Defoliation eventually occurs. New leaves are then formed which may also burn and defoliate. This will continue until the plant either dies or the chemical is no longer present in a toxic level in the root zone. Equipment used to apply herbicides should be well marked and should not be used for disease or insect control purposes. Some trees and certain ornamentals are sensitive to herbicides that are blended with fertilizers.

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